1. Field of the Invention
The present invention relates to an antenna apparatus that communicates with another apparatus with an electromagnetic signal and is preferably for use in an RFID system and a short-range wireless communication system, and a communication terminal including the antenna apparatus.
2. Description of the Related Art
In RFID systems and short-range wireless communication systems that have been increasingly used in recent years, in order to allow a mobile electronic apparatus such as a mobile telephone to communicate with another mobile electronic apparatus or a reader/writer, communication antennas are included in these mobile electronic apparatuses and the reader/writer. Japanese Unexamined Patent Application Publication No. 2006-270681 discloses an antenna included in a mobile electronic apparatus.
FIG. 1 is a diagram illustrating a manner in which a mobile information terminal 21 disclosed in Japanese Unexamined Patent Application Publication No. 2006-270681 and a reader/writer communicate with each other when they are in close proximity to each other. In an example shown in FIG. 1, a portion of a magnetic field H of an electromagnetic wave emitted from a transmission/receiving antenna portion 26 in the reader/writer is reflected or absorbed under the effect of a metal body such as a battery pack 25 in a terminal body 22 and is attenuated. A metal layer 30 is nearer to an electromagnetic wave enter side than a communication surface CS of an antenna module 10. When a magnetic field is externally applied to the surface of the metal layer 30, an induced current (eddy current) occurs on the surface of the metal layer 30 and a magnetic field H1 is generated. The magnetic field H1 generates an induced current at an antenna coil 15 in the antenna module 10.
In this example, by disposing the metal layer 30 so that it covers a portion of the antenna coil 15 and faces the antenna module 10 in close proximity to the antenna module 10, the transmission/receiving antenna portion 26 in the reader/writer and the antenna coil 15 in the antenna module 10 are inductively coupled via the magnetic field component H1 generated at the metal layer 30.
The antenna apparatus illustrated in FIG. 1 solves a problem that a communication characteristic is markedly changed by a misalignment between the center of an antenna included therein and the center of an antenna included in a communication partner when these antennas are very close to each other. In order to prevent a magnetic flux being linked to the antenna coil 15 in the antenna module 10 in the mobile information terminal 21 and the transmission/receiving antenna portion 26 in the reader/writer from being interrupted by a metal body such as the battery pack 25, the metal layer 30 is arranged to generate the magnetic flux. Accordingly, depending on the position of an interrupter such as the battery pack 25, a significant effect may not be obtained.
Furthermore, the metal layer 30 may not be effective in increasing a communication range when an antenna in an antenna apparatus and an antenna in a communication partner are spaced apart from each other.